In approximately 2 weeks, Pascal Hubert will deliver (on October 20 at Amphithéâtre Hermite of Institut Henri Poincaré (IHP) at 16h00) his “séminaire Bourbaki” lecture on this article of A. Eskin, M. Kontsevich and A. Zorich on the sums of Lyapunov exponents of the Kontsevich-Zorich cocycle with respect to -invariant measures.

The goal of today’s post is to accomplish (below the fold) a promise made here of commenting on a few aspects of Eskin-Kontsevich-Zorich paper as an *informal preparation* to Pascal’s talk.

Remark 1Actually, this survey is the main reason why I temporarily stopped the series “Lyapunov spectrum of the Kontsevich-Zorich cocycle on the Hodge bundle of square-tiled cyclic covers” at its fourth installment: indeed, similarly to the “Homoclinic/heteroclinic bifurcations” series, I decided that it would be better to continue the series only when the corresponding survey text was ready because this would allow me to eventually post “expanded versions” of the sections of the survey article.In this post we will use some parts of the followingpreliminaryversion of a survey by G. Forni and myself (currently with 150 pages) corresponding to our “Bedlewo lectures” at Banach center last year (to be submitted to the proceedings of the 2009 and 2011 editions of the School and Workshop “Modern Dynamics and its Interaction with Analysis, Geometry and Number Theory” after Giovanni and I finish correcting it).

In particular, since the survey with Giovanni is almost finished, I hope to come back the “Lyapunov spectrum of the KZ cocycle on the Hodge bundle of square-tiled cyclic covers” after today’s post.

**1. Introduction **

The so-called *Kontsevich-Zorich cocycle* over the *Teichmüller flow* on the moduli space of unit area Abelian (and/or quadratic) differentials on Riemann surfaces is an object that we discussed several times in this blog (see, e.g., this series of posts here). In particular, we will *not* (re-)explain today to the reader the notions of Abelian differentials on Riemann surfaces, translation surfaces, moduli spaces of Abelian differentials and their strata, -action and Teichmüller flow, Hodge bundle and Kontsevich-Zorich cocycle.

On the other hand, let us try to “convince” the reader that the Kontsevich-Zorich cocycle (KZ cocycle) is a natural object by briefly mentioning some of its applications:

- From the point of Dynamical Systems and Ergodic Theory, the Kontsevich-Zorich cocycle is a
*renormalization*dynamics for interval exchange transformations, translation flows and billiards. In particular, the Lyapunov exponents of KZ cocycle drive the deviations of ergodic averages of typical interval exchange transformations (after the works of A. Zorich and G. Forni in 1994 and 2001, resp.), the rate of diffusion of trajectories in Ehrenfest wind-tree model of Lorenz gases (after the work of V. Delecroix, P. Hubert and S. Lelièvre in 2011), and, to the best of my knowledge, the KZ cocycle provides one of the rare explicit examples of dynamical cocycles whose neutral Oseledets bundle is genuinely measurable (i.e., not continuous). - From the point of view of Algebraic Geometry, the techniques used to compute Lyapunov exponents of the Kontsevich-Zorich cocycle were recently employed by A. Kappes and M. Möller to classify the commensurability classes of all currently known ball quotients.

Once we believe that the Kontsevich-Zorich cocycle is a fundamental object, let’s state the main formula in Eskin-Kontsevich-Zorich paper.

We start by considering a connected component of a stratum of the moduli space of unit area Abelian differentials on a Riemann surface . From the intimate relationship between Abelian differentials and translation surfaces, we know that the natural (*non-holomorphic*!) action of on “induces” a natural *non-trivial* -action on . In this language, we recall that the *Teichmüller flow* is simply the action of the diagonal subgroup of .

The Hodge bundle over is the vector bundle whose fiber over is the first real cohomology group .

Remark 2In fact, depending on the purposes, one may wish to consider the first real homology group , the first complex homology group or the first complex cohomology group as fibers of the Hodge bundle. In general, there is no big difference between these “Hodge bundles” because and are in duality and .

In this setting, the *Kontsevich-Zorich cocycle* is the dynamical cocycle on the Hodge bundle over the Teichmüller flow obtained by transporting cohomology classes along Teichmüller flow orbits with the aid of the Gauss-Manin connection (for a more concrete description of KZ cocycle, see, e.g., this post here).

Remark 3In order to be completely honest, let us point out that the Hodge bundle is not a vector bundle in the usual sense in Dynamical Systems (i.e., a vector bundle over a manifold) because the “orbifoldic nature” of moduli spaces. In particular, the KZ cocycle is not a dynamical cocycle (in general), but rather an orbifold cocycle in the sense that the linear transformations on the fibers of the Hodge bundle are defined up to a finite group (coming from the automorphisms of some Riemann surfaces)!

Today, we’ll be interested in the Lyapunov exponents of KZ cocycle. It is not hard to check from the definitions that KZ cocycle is *symplectic* with respect to the natural intersection form on the -dimensional vector space (where is the genus of ). By Oseledets theorem, it follows that the Lyapunov spectrum of KZ cocycle (i.e., the collection of its Lyapunov exponents) with respect to any ergodic -invariant probabilty such that is -integrable (i.e., for some measurable family of norms on the fibers of ) has the form

Remark 4By Remark 3, the “usual” statement of Oseledets theorem formally doesn’t apply to KZ cocycle in general because it is not a standard dynamical cocycle. However, since KZ cocycle differs from an usual dynamical cocycle by a finite group, it is not hard to see that the Lyapunov exponents still make sense and they are well-defined for the KZ cocycle.

Remark 5As we will see later in this post (see Remark 9), there exists a continuous (family of) norm(s) on the fibers of called Hodge norm(s) such that . In particular, we have that is -integrable with respect to any -invariant probability . In other words, the -integrability of hypothesis in Oseledets theorem is “automatically” satisfied in the case of KZ cocycle.

In their article, A. Eskin, M. Kontsevich and A. Zorich showed the following fundamental result for the sum of the non-negative Lyapunov exponents of an ergodic –*invariant* probability :

Theorem 1 (Eskin-Kontsevich-Zorich)Let be an ergodic -invariant probability on a connected component of a stratum of the moduli space of unit area Abelian differentials of genus . Then,

where is a geometric quantity called Siegel-Veech constant of .

Remark 6Again, in order to be completely honest, we must say that, in their article, A. Eskin, M. Kontsevich and A. Zorich stated their result only for the ergodic -invariant probabilities that are “affine” and “regular”. As it turns out, A. Eskin and M. Mirzakhani recently proved (the “Ratner-like theorem”) that all ergodic -invariant probabilities are affine. Furthermore, all known ergodic -invariant probabilities are regular. In any event, in this post we’ll never use the “affine” and “regular” assumptions, so that we’ll forget about this remark right away.

Remark 7There is an analog formula (also due to Eskin-Kontsevich-Zorich) for the case of quadratic differentials. However, for the sake of simplicity, we’ll deal exclusively with Abelian differentials here.

Of course, this statement of Eskin-Kontsevich-Zorich theorem (EKZ theorem for short) is incomplete because we don’t know yet the definition of the Siegel-Veech constant . In principle, we could give it right now, but we’ll do so only in the *last* section of this post. Here, our “motivation” for doing so is the following one. Very roughly speaking, the *first step* towards the proof of EKZ theorem is the proof of the so-called *Kontsevich-Forni formula*, a result presenting as an *integral*

of a certain function on with respect to . As it turns out, for this first step (i.e., Kontsevich-Forni formula), one *doesn’t*need the Siegel-Veech constants and, in fact, these quantities are relevant only when one wants to convert the integral into a more tractable object (i.e., when one wants to actually compute the integral). Thus, for this reason, we will divide this post as follows: in the next section, we will state and sketch the proof of Kontsevich-Forni formula, and in the last section we will define the Siegel-Veech constants and give some applications of EKZ theorem.

Remark 8In some sense, the Kontsevich-Forni formula “explains” why the right-hand side of EKZ formula has two contributions ( and ). Indeed, if the moduli space (or rather ) were compact, an application of Stokes theorem and analytic Grothendieck-Riemann-Roch formula would show that the integral equals . However, the non-compactness of makes things more complicated because, besides , we get a “boundary” contribution that, after a lot of work, one can show that is equal to the “geometrically significant quantity” .

**2. Kontsevich-Forni formula **

By definition, the task of studying Lyapunov exponents consists precisely in understanding the growth of norm of vectors. Of course, the particular choice of norm doesn’t affect the values of Lyapunov exponents (essentially because two norms on a finite-dimensional vector space are equivalent), but for the sake of our discussion it will be convenient to work with the so-called *Hodge norm*.

** 2.1. Hodge norm on the Hodge bundle over **

Let be a Riemann surface. The *Hodge (intersection) form* on is given

for each .

The Hodge form is positive-definite on the space of holomorphic -forms on , and negative-definite on the space of anti-holomorphic -forms on . For instance, given a holomorphic -form , we can locally write , so that

Since is an area form on and , we get that .

In particular, since , and and are -dimensional complex vector spaces, one has that the Hodge form is an Hermitian form of signature on .

The Hodge form induces an Hermitian form (also called Hodge form and denoted by ) on the *complex* Hodge bundle .

The so-called *Hodge representation theorem* says that any *real* cohomology class is the real part of an *unique* holomorphic -form , i.e., . In particular, one can use the Hodge form to induce an inner product on via:

for each .

Again, this induces an inner product and a norm on the *real* Hodge bundle . In the literature, is the *Hodge inner product* and is the *Hodge norm* on the real Hodge bundle.

Observe that, in general, the subspaces and are *not* equivariant with respect to the (natural complex version of the) KZ cocycle (on ), and this is one of the reasons why the Hodge norm is *not* preserved by the KZ cocycle in general. In the next subsection, we will study *first variation formulas* for the Hodge norm along the KZ cocycle and its applications to the Teichmüller flow.

** 2.2. st variation of Hodge norm **

Let a vector in the fiber of the real Hodge bundle over . Denote by the holomorphic -forms with . By applying the Teichmüller flow to , we endow with a new Riemann surface structure such that is an Abelian differential. In particular, where is a holomorphic -form with respect to the new Riemann surface structure associated to .

Of course, by definition, KZ cocycle acts by parallel transport (with respect to Gauss-Manin connection) on the Hodge bundle, so that the cohomology classes are not “changing”. However, since the representatives we use to “measure” the “size” (Hodge norm) of are changing, it is an interesting (and natural) problem to know how fast the Hodge norm changes along KZ cocycle, or, equivalently, to compute the *first variation* of the Hodge norm along KZ cocycle:

where is the projection in the fibers of the Hodge bundle and is the Hodge norm with respect to the Riemann surface structure induced by .

In this subsection we will calculate this quantity by following (Section 2 of) the original article of G. Forni (see also the recent survey by G. Forni, A. Zorich and myself where a differential-geometric interpretations of Giovanni’s arguments are given).

By working locally outside the zeroes of , we can choose local holomorphic coordinates with . Now, we note that, by definition of the Teichmüller flow, , where , so that

Next, we write , so that we find smooth family with . By writing , and by taking derivatives, we have locally

In particular, since , we find that .

Finally, we can (locally) compute

In resume, we proved the following st variation formula (originally from Lemma 2.1′ of Forni’s 2002 paper):

Theorem 2 (G. Forni)Let be an Abelian differential and . Denote by the holomorphic (with respect to ) -form with . Then,

where .

In order to aleviate the notation, we put where is the unique -holomorphic -form with . Observe that is a *complex-valued* bilinear form.

In particular,

*Proof:* The first statement of this corollary follows from the main formula in Theorem 2, while the second statement follows from an application of Cauchy-Schwarz inequality:

Remark 9This corollary implies that the KZ cocycle is -bounded with respect to the Hodge norm, that is, for all with . Hence, given any finite mass measure on , we have that

Corollary 4Let be any -invariant ergodic probability on . Then, .

*Proof:* By Corollary 3, we have that . Moreover, since the Teichmüller flow , we have that the -invariant -plane contributes with Lyapunov exponents . In particular, .

Now, we note that is -invariant because the KZ cocycle is symplectic with respect to the intersection form on and is the symplectic orthogonal of the (symplectic) -plane . Therefore, is the largest Lyapunov exponent of the restriction of KZ cocycle to .

In order to estimate , we observe that, for any ,

by Corollary 3. Hence, by integration,

By Oseledets theorem and Birkhoff’s theorem, for -almost every , we obtain that

This reduces the task of proving that to show that for *every* . Here, we proceed by contradiction. Assume that for some Abelian differential . By definition, this means that

for some . In other words, by looking at the proof of Corollary 3, we have a case of equality in an estimate derived from Cauchy-Schwarz inequality. It follows that, by denoting the -holomorphic -form with , the functions and differ by a multiplicative constant , i.e.,

Since is a meromorphic function and, *a fortiori*, is an anti-meromorphic function, this is only possible when is a *constant*function, that is, . In particular, , a contradiction with the fact that .

By a careful inspection of the proof of the previous corollary, we saw that , i.e., the second Lyapunov exponent is naturally bounded by the integral of a certain (continuous) function on . In some sense, this is a “preview” of the Kontsevich-Zorich formula (compare with (1)) that we will state in the next subsection.

Remark 10At this stage, one could work more to derive further applications of the Hodge norm to Teichmüller dynamics: for instance, using the Hodge norm it is possible to show some uniform hyperbolicity and quantitative recurrence estimates for the Teichmüller flow with respect to any compact set , and this information was used by J. Athreya and G. Forni to study deviations of ergodic averages for billiards on rational polygons. However, we will refrain ourselves from doing so because it would lead us too far away from Kontsevich-Forni formula.

** 2.3. nd variation of Hodge norm and Kontsevich-Zorich formula **

Geometrically, is *essentially* the *second fundamental form* (or *Kodaira-Spencer map*) of the holomorphic subbundle of the complex Hodge bundle equipped with the Gauss-Manin connection. Roughly speaking, recall that the second fundamental form is where is the -component of . See the figure below.

In this language, it is possible to show that where is the Hodge form. See, e.g., this survey for more discussion on this differential-geometrical interpretation of . (A word of *caution*: the second fundamental form considered herediffers from by a sign, i.e., !)

Next, by taking a Hodge-orthonormal basis of , we have a matrix , , associated to . Define . The eigenvalues of have the form where is an eigenvalue of , i.e., induces a positive semi-definite form on . As it turns out, is essentially the *curvature form* of the holomorphic subbundle of the complex Hodge bundle equipped with the Gauss-Manin connection (see this survey for more details), i.e., the matrix also a *differential-geometrical* interpretation (similarly to ). In particular, this geometrical interpretation *hints* that *should* naturally enter into *second* variation formulas for the Hodge norm (of course, this should be compared with the fact that naturally enters into *first* variation formulas for the Hodge norm) and, *a fortiori*, the eigenvalues of *should* provide nice consequences to the study of Lyapunov exponents. In fact, as it was proposed by M. Kontsevich and proved by G. Forni in 2002, one can relate the eigenvalue of to Lyapunov exponents of KZ cocycle via the following formula:

Theorem 5 (M. Kontsevich, G. Forni)Let be a -invariant -ergodic probability on a connected component of some stratum of . Then, one has the following formula for the sum of non-negative Lyapunov exponents of KZ cocycle with respect to :

Remark 11Since , one can use the argument (Cauchy-Schwarz inequality) of the proof of Corollary 3 to see that for all . In particular, since , one can rewrite the formula above as

Remark 12Note that there is animportantdifference in the hypothesis of Theorem 2 and Theorem 5 is: in the former isany-invariant while in the latter is -invariant!

Let’s give now an outline of proof of Theorem 5. Given , let

where is a -dimensional *isotropic* subspace of the real Hodge bundle and is *any*Hodge-orthonormal basis of .

Remark 13Of course, it is implicit here that the expression

doesn’t depend on the choice of Hodge-orthonormal basis but only on the isotropic subspace . We leave this verification as an exercise to the reader.

In the sequel, we will use the following three lemmas (see Forni’s 2002 paper or this survey for proofs and more details).

Lemma 1 (Lemma 5.2′ ofForni’s 2002 paper).Let be any Hodge-orthonormal completion of into basis of a Lagrangian subspace of . Then,

Remark 14 (M. Kontsevich’s fundamental remark)In the extremal case , the right-hand side of the previous equalitydoesn’tdepend on the Lagrangian subspace :

This fundamental observation (that is hard to overestimate!) of Maxim Kontsevich lies at the heart of the main formula of Theorem 5

It is not hard to see that the notion of Hodge norm on vectors can be extended to any polyvector coming from a (Hodge-orthonormal) basis of an isotropic subspace . By slightly abusing of the notation, we will denote by the Hodge norm of such a polyvector.

Note that the Hodge norm depends only on the complex structure, so that whenever . In particular, it makes sense to consider the Hodge norm over the *Teichmüller disk* . For subsequent use, we denote by the hyperbolic (leafwise) Laplacian on (here, we’re taking advantage of the fact that is isomorphic to Poincaré’s hyperbolic disk ).

Lemma 2 (Lemma 5.2 of Forni’s 2002 paper).One has .

Finally, in order to connect the previous two lemmas with Oseledets theorem (and Lyapunov exponents), one needs the following fact about hyperbolic geometry:

Lemma 3 (Lemma 3.1 of Forni’s 2002 paper).Let be a smooth function. Then,

where , are polar coordinates on Poincaré’s disk, is the disk of radius centered at the origin and is Poincaré’s area form on .

Next, the idea to derive Theorem 5 from the previous three lemmas is the following. Denote by , and, given , for , let . In plain terms, is measuring how the (Hodge norm) size of the polyvector changes along the Teichmüller disk of . In particular, as we’re going to see in a moment, it is not surprising that has “something to do” with Lyapunov exponents.

By Lemma 3, one has

Then, by integrating with respect to the -variable in the interval and by using Lemma 2 for the computation of , one deduces

At this point, by taking an average with respect to and *using the -invariance *of to get rid of the integration with respect to , we deduce that

Now, we observe that:

- by Oseledets theorem, for a “generic” isotropic subspace and -almost every , one has that converges to as (recall that, by definition, the function is measuring the growth (in Hodge norm) of the polyvector along the Teichmüller orbit ), and
- by Remark 14, for , is
*independent*on .

So, for , this discussion (combined with an application of Lebesgue dominated convergence theorem and the fact that as . See, e.g., this survey for more details) allows to show that

This completes the sketch of proof of Theorem 5.

Remark 15Essentially the same argument above allows to derive formulas forpartial sumsof Lyapunov exponents. More precisely, given a -invariant -ergodic probability with (for some ), one has

where is the Oseledets subspace associated to the top Lyapunov exponents.In general, this formula is harder to use than Theorem 5 because the right-hand side of the former implicitly assumes somea prioricontrol of while the right-hand side of the latter is independent of Lagrangian subspaces (as noticed by M. Kontsevich).

Remark 16Actually, for the “integration by parts” argument relevant for the EKZ formula, the most “convenient” form of Kontsevich-Forni formula is

where is a Hodge-orthogonal basis of a Lagrangian subspace of .

Now, let us complete the discussion of today’s post by making some comments in the next (final) section on Siegel-Veech constants and some applications of EKZ formula.

**3. Siegel-Veech constants and EKZ formula **

** 3.1. Siegel-Veech constants **

Given an Abelian differential , we consider the associated translation surface. Given a closed regular geodesic in a translation surface , we can form a *maximal cylinder* by collecting all closed geodesics of parallel to not meeting any zero of . In particular, the boundary of contains zeroes of . Given a maximal cylinder , we denote by its *width* (i.e., the length of its waist curve ) and by its *height* (i.e., the distance across ). For example, in the figure below we illustrate two closed geodesics and (in the horizontal direction) and the two corresponding maximal cylinders and of a L-shaped square-tiled surface (see these posts here and here for more explanations). In this picture, we see that has width , has width , and both and have height .

Definition 6Let be a translation surface. Given , we define

Informally, *counts* the fraction of the area of the translation surface occupied by maximal horizontal cylinders of width bounded by .

Of course, the quantity depends a *lot* on the geometry of and the real number . However, W. Veech and Ya. Vorobets discovered that given any -invariant -ergodic probability , the quantity

*doesn’t* depend on . In the literature, is called the *Siegel-Veech constant*of .

Remark 17Our choice of normalization of the quantity leading to the Siegel-Veech constant here isnotthe same of Eskin-Kontsevich-Zorich. Indeed, what they call Siegel-Veech constant is in our notation. Of course, there is no conceptual different between these normalizations, but we prefer to take a different convention from Eskin-Kontsevich-Zorich because appears more “naturally” in the statement of EKZ formula.

Remark 18It is not hard to see from the definition that Siegel-Veech constants are alwayspositive, i.e., for any -invariant -ergodic .

The Siegel-Veech constants of Masur-Veech measures were computed by A. Eskin, H. Masur and A. Zorich and they are intimately related to volumes of strata calculated by A. Eskin and A. Okounkov. As an outcome of these works, it follows that the Siegel-Veech constants of Masur-Veech measures are *rational numbers*. In particular, by combining this fact with EKZ formula, we deduce that:

Corollary 7 (Eskin-Kontsevich-Zorich)For the Masur-Veech measures , one has

Note that this corollary is quite spectacular because Lyapunov exponents normally are quantities coming from merely *measurable subbundles* and there is no a priori reason to believe in their rationality!

Actually, by looking at EKZ formula and taking the case of Masur-Veech measures as a prototype, it is tempting to conjecture that the sums of Lyapunov exponents of KZ cocycle with respect to -invariant probabilities are always rational. In this direction, let us point out that the following result of Eskin-Kontsevich-Zorich allowing to compute (and show the rationality of) Siegel-Veech constants of measures coming from *square-tiled surfaces*.

Let be a square-tiled surface, i.e., comes from a finite covering branched only at . Since is the stabilizer of in (when the periods of generate the lattice ), the -orbit of give all square-tiled surfaces in the -orbit of . Moreover, since the Veech group is a finite-index subgroup of , one has

where .

In this context, for each , we write where are the maximal horizontal cylinders of , and we denote the width and height of by and .

Theorem 8 (Eskin-Kontsevich-Zorich)The Siegel-Veech constant of the -invariant -ergodic probability supported on the -orbit of the square-tiled surface is

In particular, the sum of Lyapunov exponents of such a measure satisfies

Remark 19Actually, the rationality result in the previous theorem is part of a slightly more general result of Eskin-Kontsevich-Zorich. Indeed, they showed that the Kontsevich-Forni formula implies that the sums of Lyapunov exponents of KZ cocycle with respect to measures supported on Teichmüller curves (i.e., closed -orbits) are essentially given by the orbifold degree of the determinant line bundle of the Hodge bundle over these Teichmüller curves (a rational number as any orbifold degree of a line bundle). In particular, this generalizes the case of square-tiled surfaces because it is known that their -orbits are closed.

For example, the picture below illustrates the computation of the -orbit of a -shaped square-tiled surface with squares (shown in the middle of the picture):

Here, we’re using the fact that the group is generated by the matrices and , so that -orbits of square-tiled surfaces can be determined by successive applications of and .

From the picture we infer that and

- where are horizontal maximal cylinders with and , ;
- where is a horizontal cylinder of heigth and width ;
- where are horizontal maximal cylinders with and ,

By plugging this into Theorem 8, we get that the Siegel-Veech constant of the -invariant probability supported on is

** 3.2. Some of consequences of EKZ formula **

Let’s now apply Eskin-Kontsevich-Zorich formula to show the positivity of some Lyapunov exponents of KZ cocycle in the *higher* genus case:

Proposition 9 (Eskin-Kontsevich-Zorich)Let be a ergodic -invariant probability measure on a connected component of a stratum of unit area Abelian differentials of genus . Then,

(and, actually, ).

*Proof:* Since , it suffices to show that the right-hand side of Eskin-Kontsevich-Zorich formula is to get that , and this follows from the computation

based on the non-negativity of the Siegel-Veech constant and the assumption .

Another interesting consequence of EKZ formula is the following. A. Eskin, M. Kontsevich and A. Zorich also showed in their article a *version* of the their formula for *quadratic differentials*, and they used it to compute Siegel-Veech constants of -invariant -ergodic probabilities supported in the *hyperelliptic connected components* and of the strata and . The outcome of their computation is the fact that Siegel-Veech constant of *any* such -invariant -ergodic is

Hence, by combining this with EKZ formula, we conclude that

Theorem 10The sum of Lyapunov exponents is

In particular, since the sole two strata and in genus are hyperelliptic connected components, one has that, for any -invariant -ergodic ,

because . This fact was conjectured by M. Kontsevich and A. Zorich, and it was firstly demonstrated by M. Bainbridge a few years before the Eskin-Kontsevich-Zorich article was available.

Remark 20In a very recent work, D. Aulicino further studied the problem of classifying -invariant measures with totally degenerate spectrum from the point of view of theTeichmüller disks(i.e., -orbits) contained in therank-one locus. More precisely, following G. Forni, we define therank–locusof the moduli space of Abelian differentials of genus is . Note that . In the literature, the locus is sometimes calleddeterminant locus(because ). Observe that these loci are naturally related to the study of Lyapunov exponents of KZ cocycle: for instance, by Theorem 5, any -invariant probability with has totally degenerate spectrum. In his work, D. Aulicino showed that there arenoTeichmüllerdiskscontained in for or , the so-called Eierlegende Wollmilchsau and Ornithorynque are the sole Teichmüllerdiskscontained in and , and, furthermore, if there are no Teichmüllercurvescontained in , then there are no Teichmüllerdiskscontained in for . It is worth to point out that Teichmüller disks are more general objects than regular affine measures, so that Proposition 9 doesn’t allow to recover the results of D. Aulicino.

Remark 21The Eierlegende Wollmilchsau and Ornithorynque are two square-tiled surfaces with the following properties:

- Eierlegende Wollmilchsau lives in and it is decomposed in two maximal horizontal cylinders , with and , and -orbit is a singleton;
- Ornithorynque lives in and it is decomposed in two maximal horizontal cylinders , with and and its -orbit is a singleton.

By plugging these facts into Theorem 8, one can compute the Siegel-Veech constants of the measures and associated to these examples, and then, by EKZ formula, one can calculate the sum of their Lyapunov exponents. By doing so, one finds:

and

Since for any -invariant ergodic , one concludes that and , a fact that known (by other methods) from results of Forni and his collaborator.

Remark 22In the case of coming from square-tiled surfaces , the formula in Theorem 8 for combined with EKZ formula suggests that one can write downIndeed, firstly we observe that square-tiled surfaces are intimately related to pairs of permutations : a pair of permutations gives rise to a square-tiled surface with squares by taking unit squares , , and by gluing (by translations) the rightmost vertical side of to the leftmost vertical side of and the topmost horizontal side of to the bottommost horizontal side of . Of course, by renumbering the squares of a given square-tiled surface we may end up with different pairs of permutations, so that a square-tiled surface determines a pair modulo simultaneous conjugation, i.e., modulo the equivalence relation if and only if and for some . Finally, it is possible to check that the action of the matrices and on square-tiled surfaces translation into the action and on pairs of permutations. Secondly, in this language, the heights and widths of its horizontal cylinders are determined by the cycles of the permutation . So, by recalling that and generate , we can use the right-hand side of the formula in Theorem 8 to convert the computation of the Siegel-Veech constant of the -invariant probability supported on into a combinatorial calculation with pairs of permutations that an adequate computer program can perform.computer programsto calculate the sum of Lyapunov exponents.

In fact, such computer programs forMathematicaandSAGEwere written by, e.g., A. Zorich and V. Delecroix, and it is likely that they will be publicly available soon.

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